1. Field of the Invention
The invention relates to a process that produces improved adhesion of a relatively heavy protective conversion coating on a metal surface, with increased conversion coating weight, by applying to the surface a pretreatment composition comprising a derivative of poly(alkenylphenol} before applying the protective conversion coating. The process of the invention is especially useful in forming chromium and silica containing conversion coatings on aluminum surfaces, especially those that have been continuously cast.
2. Statement of Related Art
The need for applying protective conversion coatings to metal surfaces has long been recognized in the art. Important uses of such conversion coatings are to prevent corrosion and to serve as a base for subsequent painting of the metal surface. The corrosion quality of the painted article is determined by the level of adhesion of the paint to the protective conversion coating and/or the adhesion of the latter to the metal substrate, as well as by the resistance of the base coated and painted metal surface to humidity, salt-spray and similar tests. Where a forming operation is to be employed subsequent to painting, the protective conversion coating must provide a satisfactory level of paint adhesion during the forming step and of corrosion resistance of the metal in the environment in which it is to be used.
In the past, difficulties have often been encountered in developing a process for applying a protective conversion coating to metal surfaces and particularly to continuously cast aluminum surfaces. These difficulties result from the requirement that a protective base-coated product, after being painted, must exhibit satisfactory formability, adhesion, and corrosion characteristics and the fact that prior art methods generally require a compromise between the requirements of adhesion and corrosion resistance, particularly on continuously cast aluminum surfaces: Conversion coating weights greater than about 75 milligrams per square meter (hereinafter "mg/m.sup.2 "), are desirable on aluminum surfaces, and especially on continuous cast aluminum surfaces, to provide adequate corrosion resistance to the surfaces. For the best corrosion resistance, the conversion coating weights are preferably in the range from 86-130 mg/m.sup.2, more preferably in the upper half of this range. However, it has been found necessary to keep the coating weight of the above-described hexavalent chromium protective conversion coating on continuously cast aluminum surfaces generally less than 75 mg/m.sup.2, in order to obtain adequate adhesion of paint to the conversion coating.
U.S. Pat. Nos. 4,443,015, 4,457,790, 4,517,028, and 4,963,596 all describe the treatment of metal surfaces with dilute solutions of poly{alkenylphenol} derivatives and more specifically of a poly{4-vinylphenol} derivative or an acid salt of a poly{4-vinylphenol} derivative. These treatment solutions are described as an alternative to the use of hexavalent chromium compound-containing solutions for the acidic post-treatment of phosphatized and or conversion coated metal surfaces. The solutions of these patents may also be employed on uncoated metal surfaces as an alternative to phosphating or other conversion coatings with the object of improving the corrosion resistance and paint adhesion characteristics of the metal surface.
U.S. Pat. No. 4,647,316 describes a hexavalent chromium protective conversion coating composition also comprising silica or silicates or mixtures thereof and phosphate and a process for coating metal surfaces therewith. There is nothing in the disclosure of this patent which would tend to suggest or otherwise provide motivation for applying a pretreatment solution of the type described herein to the metal surface before applying the protective conversion coating.